The invention relates to a process for drying coal by means of a two-stage process involving flow-through circulation heating elements.
It is known to effect the drying in the first stage and the further heating in the second stage by means of two flow-through circulation heaters which are operated by means of combustion gases. This permits to obtain a favorable thermal efficiency. The temperature of the combustion gases at the entrance to the second stage must be kept low at about 550.degree. in order to prevent a treatment which would affect the quality of the coal. At higher entrance temperatures there is danger of an interference with the coking properties of the dried coal. There may even start an initial degasification or a certain pyrolytic modification of individual carbon grains.
On the other hand it is a definite disadvantage if the temperature of the vapors received from the first stage is close to the condensation point of the steam since the then possible drop formation in case of continuous operation may cause damages to the rotor of the blower since before the vapors are permitted to pass into the atmosphere they must go through a blower and subsequently through a dust separator, for instance an electrofilter or a cloth filter. In these dust separators the water drop formation can also have undesirable effects, for instance deposits of extremely finely divided carbon which are very difficult to remove or crust formation at the filter plates of the electrofilter or of the filter cloths.
Particular difficulties can arise if sulfur-containing gases are subjected to combustion in the combustion chamber since in that case the combustion gas, and therefore also the emanating vapors will contain a certain amount of sulfur dioxide. Since the sulfuric acid condensation point is between about 170.degree. and 180.degree. C. there forms sulfuric acid, particularly in the blower and dust separator already below this temperature range.
It has previously been proposed to prevent a comparatively low vapor exit temperature in case of two-stage flow-through circulation heating elements by reducing the coal throughput through the circulation heaters. This artifice, however, interfers with the economics of the process.
The present invention therefore has the object to even improve the throughput of coal in case of two-stage flow-through circulation heaters without reaching the dangerous temperature range below the dewpoint formation and below the condensation point of sulfuric acid.